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Found 11 results

1. ## Complex Eigenvalue Analysis

Hello, i need some help because i cant find any solution to my problem. I've a Project where i have to work with dynamic stiffness (pbusht element) of a structure and want to see the Eigenvalues and normal modes of my structure. To get into the theory i set up a simple model: A CBUSH element with a pbush property and with a 1D Mass (m=1kg) on top and a BC on the bottom. I specified in pbusht property with elastic stiffness of K=100 (N/mm) and a loss factor of GE=0.3 Then i did an normal mode analysis and i know that the "Damping is neglected" (see link (1)) and i have just my K=100. The solution of my normal mode analysis is a normal mode frequency of omega = 50,33 Hz. This is correct, because the formula for free, undamped oscillations is f = sqrt(k/m)/2PI = sqrt(100 N/mm / 0,001 tonnen) / 2PI = 50,33 Hz (i use N, mm, tons as units) Now i want to include the damping, so i use the complex eigenvalue analysis. In theory i learned that for free, undamped oscillations with the damping factor D<1 the cycle frequency omega_d = omega * sqrt(1-D²). Because of the relation that 2*D = tan(phi) = GE i can fill in the my loss factor of GE=0.3. Then i got my omega_d = 49.76 Hz. This is also logically, because for D<1 the oscillations of damped systems are always slower than the ones of the undamped system. ...so omega_d < omega is okay. Now i did the complex eigenvalue analysis (just defined a load collector EIGC card and connected it to the CMETHOD of the complex eigenvalue load step (of course the SPC and EIGRL card to the SPC and Method(Struct) too like i did for the normal modes). The result of the simulation is a frequency of 50,88 Hz and a loss factor of 0,2935. Now i'm confused - first: why is my loss factor not 0,3? Yes, its just a little difference but its a really simple model - in the normal modes analysis the eigenfreqency was also exact to 5 decimal places. But okay, maybe some numeric accuracy. But i definitely do not understand the frequency of 50,88 Hz. Where is the mistake? Its higher than before whats definitely wrong in theory and not my calculated value. Anyone got some ideas? I already thank you Regards Rene (1) https://altairhyperworks.com/hwhelp/Altair/2019/help/os/topics/solvers/os/analysis_normal_modes_r.htm?zoom_highlightsub=eigra (2) https://altairhyperworks.com/hwhelp/Altair/2019/help/os/topics/solvers/os/analysis_complex_eigenvalue_r.htm CBUSH.hm modal_complex.fem modal_complex.h3d modal_normal.fem modal_normal.h3d
2. ## Non-linear Modal Analysis

How to conduct Non-linear Modal analysis ?
3. ## Polar chart in HyperLaminate

Hey everybody! I've just tried HyperLaminate modul in HyperMesh and found that it can calculate the stiffness of a composite laminate, the [A] [D] matrices etc. as you can see on the attached images. Is there any tool which represent the stiffness of the laminate in polar chart? I think Ansys is capable of doing it, but I could not find this function in HyperWorks. Thank you for your answers! Kind regards, Imre
4. ## How to define the stiffness (K) of celas2 element

Hello everybody, i guess it is easy to solve but i can't define the stiffness of a celas2-element. I created 1D --> springs --> celas2, Chose 2 nodes, Chose dof (3), Chose property, but still i don't know how to apply the stiffness (K). It tried with PARAM --> ELASSTIF, but it doesn't work. Thank you in advance Greets, Fatih Uysal
5. ## Operational Modal Analysis

Is it possible to conduct Operational Modal Analysis (OMA) in Optistruct ?
6. ## Defining a stiffness of a spring damper element through a curve, any help?

Hello there, I need some advice for defining the stiffness of a spring damper element with a curve. My task is to model a 1 mass oscillator with a non-linear stiffness. But first of all I want to understand how to define a stiffness through a curve. So I tried to model a linear stiffness through the curve and verify it with the regular linear simulation. My questions are: Can anybody tell me what I have done wrong with my curve definition? I defined a sinusodial force with +/- 10kN but the mass is just swinging in the negative area? Can anybody specify what the independent variable is? What do I have to put there when my curve is defined as force over displacement? I have attached my files. I appreciate every help
7. ## contact pair v/s weld elements for modal and stiffness analysis of ladder chassis

For Modal and Stiffness(torsional and bending) of ladder frame chassis, is there any significant difference in results when I use stick contact between frame and cross-members or when I use shell/rigid weld elements to connect cross-members and long members?
8. ## Effect of RBE3s on stiffness

Hello, If I am to put RBE3 in a hole (with no force or constraint attached to the dependant node) will this imply any stiffness to the voids outer surface where the independent nodes are? Or does the hole just remain as it would if it was a void with nothing in it? What type of 1D element could be used to provide a defined stiffness to the outer surface of a hole? Thank you in advance
9. ## Stiffness calculation

Is it possible to calculate the Stiffness (N/m or N/mm) of the structure from any analysis??
10. ## Assigning Preload and stiffness simultaneously in 1D spring

Hi, Instead of rubber bellows I have used Cbush element and given stiffness to it. I want to give preload also. I have checked we cannot give preload to cbush element. CROD,CBAR,CBEAM support for giving preload. Is there any option (spring type) to give preload and stiffness to 1D element simultaneously ? Please let me know soon. Regards, Pratik
11. ## Effective Adjustment of CBUSH Stiffness

I am trying to squeeze a femur model with two sets of stiff springs top and bottom, and allow it to rotate with the other (spring BCs come from a research paper). Instead, the model bends where the mesh meets the springs that should remain mostly straight. However, I have tried adjusting all DOF stiffnesses in the PBUSH card without luck. All springs have been assigned to the local coordinate system (shown in right on photos) using CBUSH card. It seems to me I need to stiffen K2 or K6 (y-tension, z-moment), but neither of these solves the issue. Attached graphs show "TY" stresses. Femur_Model_6_26.fem
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